The brownstone blocks of Park Slope rarely appeal to modern-architecture aficionados. Yet here, on this block smack in the middle of the North Slope, sits a house with interiors renovated by the mid-century master I.M. Pei. The architect’s son, T’ing, bought the building in the mid-seventies, and his Pritzker-winning dad did the renovation of the owner’s duplex on the third and fourth floors. That apartment retains Pei’s imprint, with sleek, all-white geometric lines and two tall, slender floor-to-ceiling archways. “A student of Pei would recognize the work immediately,” says listing broker Jessica Buchman. The townhouse is also quite a bit cheaper, on a square-foot basis, than Pei’s most recent work: the Centurion, the new condo building on West 56th Street that marks his return to Manhattan residential architecture after a four-decade absence.